Optical signal time-scaling arrangement

ABSTRACT

A signal time-scaling arrangement utilises a known configuration comprising the use of a laser which has its wavelength varied over time, a modulator coupled to the laser and to a modulating analogue electrical source and a dispersion means coupled to the output of the modulator and providing at its output signal which is a stretched version of the analogue modulating a signal, but provides for the modulator to be a single-sideband modulator instead of double-sideband. The effect of this is to enable the use of a much simple laser-control system involving a CW laser output which is wavelength-ramped continuously between quite narrow wavelength limits, while allowing the use of a dispersion means having a wide dispersion characteristic in order to provide the required degree of time-scaling. The invention is application to an ADC system or to a Doppler system.

[0001] The invention relates to a signal time-scaling arrangement, andin particular, but not exclusively, a signal time-scaling arrangement asemployed in the processing of very high speed electrical signals, andmore particularly as employed in the conversion of analogue signals ofthis nature to digital form.

[0002] Conventionally, analogue electrical signals are converted intodigital form using one of the known electrical analogue-to-digitalconversion (ADC) techniques, which include, purely as examples,dual-ramp, charge-balancing, successive-approximation andflash-conversion arrangements. In all these methods there is a limit tothe rate at which digital values can be formed from the input analoguesignal. In the slower methods (virtually all except the flash converter)time must be allowed for the required integration or approximationprocesses to take place, this time being provided by a suitablesample-and-hold arrangement which freezes the input while it is beingoperated on. In the case of flash conversion, which is normally thefastest variety, although there is no sample-and-hold operation, thereare still the unavoidable propagation delays associated with the variousstages of the converter. These speed limits from whatever causedetermine the maximum sampling-rate capability of the ADC.

[0003] There is, however, a need for an ADC system which can provide anadequate number of digitised samples of an analogue signal lasting foronly a short period, e.g. of the order of nanoseconds. This has beenfound to be extremely difficult to achieve using an electrical ADC byitself and supplementary measures have been taken in the past in anattempt to approach the required performance. One such measure is basedon the use of a time-stretching technique and involves the use of alaser, a modulator, a dispersion-producing means and a conventionalelectronic ADC. This system is illustrated in FIG. 1, in which anoptical source (in practice a laser) 10 is arranged to produce radiationof different wavelengths over time between a low and a highwavelength-value limit, the thus modulated optical radiationconstituting a carrier which is then intensity-modulated in a modulator12 by an electrical analogue signal 14, the digital equivalent of whichis desired to be obtained. The resultant modulated carrier is passedthrough a dispersion means 16 after which it is converted from opticalto electrical form in a suitable converter 18 and digitised in anelectronic ADC 20. The digital representation of the signal 14 is thenprocessed as required.

[0004] Conventionally, the modulator 12 is a readily available modulatorof, for example, the Mach-Zehnder type. It has been found in practicethat the laser 10 used for this process has to be of considerablecomplexity in order to accommodate the large wavelength variationsnecessary to be able to resolve the very fast signals 14 on themodulator input. Complexity of design naturally incurs the drawback ofincreased expense.

[0005] In accordance with the invention there is provided a signaltime-scaling arrangement comprising: an optical source for deliveringradiation at a wavelength which is variable with time over a givenwavelength range; a single-sideband modulating means having a firstinput coupled to the optical source, a second input coupled to amodulating source, and an output; and a signal-dispersion means coupledto the output of the modulating means, the signal-dispersion means beingoperative to impose different delays on different wavelength componentsof the radiation, an output of the signal-dispersion means forming theoutput of the signal time-scaling arrangement.

[0006] In a second aspect of the invention, a method for time-scaling asignal comprises: providing a source of optical radiation of awavelength which is varied with time over a given wavelength range;intensity-modulating said radiation with an analogue signal which is tobe time-scaled, the intensity-modulation taking place in asingle-sideband modulator, and subjecting the modulated radiation to adispersion process whereby different wavelength components of theradiation are delayed by different amounts, the therebydispersion-processed modulated radiation constituting the time-scaledsignal.

[0007] The optical source is preferably a continuous-wave optical sourceand the variation of the radiation wavelength is preferably a continuousvariation over a given time period, more particularly a substantiallylinear ramp.

[0008] The modulating means may be fed with the modulating signal and aHilbert-transformed version of the modulating signal, the modulatingmeans may be a double-sideband modulator having a modulating input fedwith the modulating signal and an output connected to thesignal-dispersion means through a phase modulator, a modulating input ofthe phase modulator being fed with the Hilbert-transformed modulatingsignal. Alternatively, the modulating means may be a Mach-Zehndermodulator whose electrodes, which are associated with the two limbs ofthe modulator's “Y” waveguide arrangement, are fed respectively by themodulating signal and the Hilbert-transformed version of the modulatingsignal.

[0009] The signal-dispersion means may comprise an optical fibre havingintrinsic dispersion properties. The signal-dispersion means maycomprise a fibre Bragg-grating device. The optical source is, mostconveniently, a laser.

[0010] A third aspect of the invention provides for an ananalogue-to-digital conversion arrangement comprising a signaltime-scaling arrangement as described above, in which the output of thesignal-dispersion means is coupled to an opto-electrical converter forconverting the optical signal at the output of the signal-dispersionmeans into an electrical signal, and an output of the opto-electricalconverter is coupled to an electrical analogue-to-digital conversionmeans.

[0011] The opto-electrical converter is preferably a photodiode, thoughan avalanche photodiode or a photo-transistor are possible alternatives.

[0012] According to a fourth aspect of the invention, afrequency-shifter arrangement comprises a signal time-scalingarrangement as described above, in which spectral components of themodulating signal appear shifted in frequency at the output of thedispersion means. The frequency-shifter arrangement finds possible useas a Doppler generator arrangement, in which the modulating source is asource of continuous sinewaves. The sinewaves may be of a predeterminedfrequency or within a predetermined range of frequencies.

[0013] The signal time-scaling action may be either a time-stretchingone or a time-compressing one. For time-stretching the laser wavelengthmay be varied in a direction of increasing frequency and the dispersionmeans may subject the higher-frequency components of the radiation to alonger delay than the lower-frequency components of the radiation.Alternatively, the wavelength is varied in the opposite direction and itis the lower-frequency components of the radiation that are subjected tothe longer delay.

[0014] In the time-compression case, the laser wavelength may be variedin a direction of decreasing frequency and the dispersion means maysubject the higher-frequency components of the radiation to a longerdelay than the lower-frequency components of the radiation.Alternatively, the wavelength is varied in the opposite direction and itis the lower-frequency components of the radiation that are subjected tothe longer delay.

[0015] An embodiment of a signal time-scaling arrangement in accordancewith the invention will now be described by way of example only with theaid of the drawings, of which:

[0016]FIG. 1 is a schematic diagram of an ADC arrangement incorporatinga known time-scaling arrangement;

[0017]FIGS. 2 and 3 illustrate an undesirable effect of the modulatorconventionally used in the known time-scaling arrangement;

[0018] FIGS. 4(a), (b) and (c) are waveform/timing diagrams illustratingthe benefits of the application of a time-scaling arrangement to an ADCsystem;

[0019]FIG. 5 is a schematic diagram of a first realisation of amodulator suitable for use in a time-scaling arrangement according tothe invention;

[0020]FIG. 6 is a schematic diagram of a second realisation of amodulator suitable for use in a time-scaling arrangement according tothe invention:

[0021]FIG. 7 is a diagram of a dispersion means suitable for use in atime-scaling arrangement according to the invention, and

[0022]FIG. 8 shows how the dispersion means employed in an embodiment ofthe invention may be implemented in practice.

[0023] The inventors have realised that the major problem with the usualtime-scaling system as employed in the ADC arrangement described abovelies in the nature of the modulator used. The problem will now beexplained with reference to FIGS. 2 and 3.

[0024] When a sinusoidal RF signal is applied at the electrical input,the conventionally used modulator produces at its output the typicalsignal composition associated with double-sideband (DSB) modulators,namely a high-amplitude carrier 30 and a pair of sidebands 32, 34equidistantly spaced from the carrier (see FIG. 2). The system relies onthe presence of dispersion after the modulator 12, and it is thisdispersion which causes a differential rotation of the two sidebandsrelative to each other as a function of frequency, so that ultimately afrequency is reached at which the two sidebands are equal and opposite(see the sideband vector 36 in FIG. 2), in which case there is no RFsignal detected after optoelectronic conversion. This frequency is shownas f₀ in FIG. 3. The upshot of this is that there is a maximum frequencycomponent in the modulating signal 14 which is capable of beingtransmitted without undue attenuation due to this signal-fading effect.This in turn has meant that, in order to accommodate very highfrequencies, the amount of dispersion in the dispersion means 16 has hadto be very limited, which adversely affects the degree oftime-stretching that can be achieved by the system.

[0025] A reduction in the degree of time-stretching obtained in turnimpacts the usefulness of the ADC 20 used at the output end of thesystem. This is more clearly explained with reference to FIG. 4, inwhich the time-stretched signal is shown in FIG. 4(b) against theoriginal unstretched modulating signal 14 in FIG. 4(a). The ADC 20 has amaximum sampling rate as shown in FIG. 4(c) and it can be seen how theinadequate definition of the original signal by only 1 sample isreplaced by the much better definition by 5 samples in thetime-stretched case. (The number of samples given is exemplary only).Clearly, it is required that the modulating signal be stretched as muchas possible so that an adequate definition can be derived of thehighest-frequency components likely to be encountered.

[0026] Finally, in order to recover the high-frequency performance ofthe system in relation to the practical sampling performance of the ADCused, it has been necessary to increase the range of wavelengthvariation of the laser used as the source. As mentioned earlier, thishas meant very complex laser designs with concomitant high costs.

[0027] The solution to this impasse proposed by the present inventors isto, employ instead of a DSB modulator a single-sideband (SSB) one. Theeffect of this measure is to drop one of the sidebands 32, 34 in FIG. 2,which means that, no matter how much dispersion is employed downstreamof the modulator, phase-cancellation will not take place and thereforethe wide-wavelength-range/low-dispersion tradeoff which is a feature ofthe known system can be replaced by a narrow wavelength range and a highdispersion. This in turn enables the laser system and associated controlelectronics to be greatly simplified, with resultant decrease in costs.

[0028] One realisation of an SSB modulator suitable for use in thepresent arrangement is shown in FIG. 5. In FIG. 5 the basic modulator isa conventional DSB Mach-Zehnder modulator 40 receiving on one input 42the laser output radiation Em(t) and on another input 44 the modulatingsignal v(t). The output of the modulator 40 is taken to a phasemodulator 46 which receives on a modulating input 48 a Hilbert transformof the modulating signal v(t), namely H[v(t)]. The output of the phasemodulator 46 feeds the dispersion means 16. The transform leaves theamplitudes of the spectral components of v(t) unchanged, but shiftstheir phases by π/2.

[0029] In an alternative realisation of the SSB modulator involving theHilbert transform principle, a Mach-Zehnder modulator has its drivingcharacteristics modified in order to suppress one of the sidebands. Thisis illustrated in FIG. 6, in which the electrodes 50, 52 associated withrespective limbs 54, 56 of the “Y” waveguide arrangement 58, haverespective voltages v(t) and H[v(t)] applied to them.

[0030] The dispersion means 16 is preferably constituted by a suitablydesigned optical-fibre lightguide having a core possessing inherentdispersive properties. Such are readily available and include, in fact,many fibres used in other applications which would prefer adispersion-free medium and often have to take corrective measures tocompensate for the undesired dispersion that occurs in the fibreactually used. Alternatively a device such as a fibre Bragg-grating maybe employed. One form of this is shown in FIG. 7 and comprises anoptical fibre 60 having a cladding element 62 surrounding an inner core64 composed of a material which is light-transmissive but which hasalong its length regions 66 of periodically varying refractive index.Such regions will generally have a sequentially increasing or decreasingperiod of refractive index variation.

[0031] The effect of such regions upon the incoming light is that thedifferent wavelength components of that light are transmitted by some ofthe regions and reflected by others so that, as a result of thedifferent path lengths involved, the different wavelength componentsarrive at different times back at the input of the fibre, where they arecollected and passed on for further processing. A practical mechanismfor achieving this is to use a circulator (see FIG. 8) which allows theincoming light to enter the fibre 60 and the reflected light to leavethe fibre 60 and exit along an output guide 68.

[0032] Production of the regions 66 may be effected by the applicationof, e.g., external ultraviolet light to the fibre.

[0033] The application of the generic time-stretching arrangement to anADC has already been covered in connection with the FIG. 1 arrangement,and the present system may likewise be deployed in such an ADC context.It is, however, also possible to use the present inventivetime-stretching system in other applications.

[0034] One such application is an RF frequency shifter, a prominent useof which would be as a Doppler generator. To make the inventivearrangement suitable for use in a Doppler system (e.g. for false targetgeneration), it is necessary simply to apply to the modulator as themodulating signal a source of sinewaves, preferably a continuous source(CW signal). Then, because whatever appears on the modulating input ofthe modulator undergoes time-scaling, that signal will appear at theoutput of the OEC 18 as a sinewave of a different frequency. Thedifference between the two frequencies can be varied by altering eitherthe wavelength-variation range of the laser 10 or the amount ofdispersion in the dispersion means 16. The use of an SSB modulator inaccordance with the present invention enables higher frequencies to becatered for than would be the case in the DSB arrangement by virtue ofthe lack of dispersion-related signal fading (see FIG. 3).

[0035] It should be noted that the above-described frequency-shiftingfunction applies not only to single-frequency sinewaves, but to anysignal having a plurality of frequency components. These components willbe changed in frequency simultaneously by this process.

[0036] It is worth noting that, in this Doppler application, thetime-scaling can be arranged to be either a time-stretching or atime-compression operation. To achieve stretching, the wavelength of thelaser can be varied in the direction of, for example, increasedfrequency, while the fibre is of the type which gives increasing delaywith increasing frequency (i.e. λ₄<λ₁ in the FIG. 7 example). For thetime-compression case the laser wavelength may be lengthened over time(i.e. decrease in frequency), with the fibre again providing increaseddelay with increased frequency. Alternatively, for stretching thewavelength could be varied in the direction of decreasing frequency,provided the dispersion was such as to increase delay in the lowerfrequencies relative to the higher frequencies, and likewise forcompression the wavelength could be varied in the direction ofincreasing frequency, provided this same dispersion characteristic held.

[0037] Time-compression produces sinewaves of increased frequencies andtime-stretching produces sinewaves of decreased frequencies relative tothe modulating input 14.

[0038] The invention envisages the use of a simple laser system andassociated control arrangement which provides a CW (continuous-wave)laser output continuously wavelength-varied, preferably in a linear, orsubstantially linear fashion, between relatively narrow wavelengthlimits. Other wavelength-modulating characteristics (e.g. logarithmic)may, however, be used if desired. It is necessary only that thecharacteristics be fully known so that the temporal behaviour of theoutput of the time-scaling arrangement can be properly interpreted. Anon-linear wavelength variation will normally result in distortion ofthe signal envelope at the output of the dispersion means, which in someapplications can be desirable.

1. Signal time-scaling arrangement comprising: an optical source fordelivering radiation at a wavelength which is variable with time over agiven wavelength range; a single-sideband modulating means having afirst input coupled to the optical source, a second input coupled to amodulating source, and an output; and a signal-dispersion means coupledto the output of the modulating means, the signal-dispersion means beingoperative to impose different delays on different wavelength componentsof the radiation, an output of the signal-dispersion means forming theoutput of the signal time-scaling arrangement.
 2. Signal time-scalingarrangement as claimed in claim 1, wherein the optical source is acontinuous-wave optical source.
 3. Signal time-scaling arrangement asclaimed in claim 2, wherein the variation of the radiation wavelength isa continuous variation over a given time period.
 4. Signal time-scalingarrangement as claimed in claim 3, wherein the variation of theradiation wavelength is a substantially linear ramp.
 5. Signaltine-scaling arrangement as claimed in claim 3 or claim 4, wherein themodulating means is fed, in use, with the modulating signal and aHilbert-transformed version of the modulating signal.
 6. Signaltime-scaling arrangement as claimed in claim 5, wherein the modulatingmeans comprises a double-sideband modulator having an input connected tothe modulating source and an output connected to the signal-dispersionmeans through a phase modulator, a modulating input of the phasemodulator receiving, in use, the Hilbert-transformed modulating signal.7. Signal time-scaling arrangement as claimed in claim 5, wherein themodulating means is a Mach-Zehnder modulator, the electrodes associatedwith the two limbs of the modulator's “Y” waveguide arrangement beingfed respectively, in use, by the modulating signal and theHilbert-transformed version of the modulating signal.
 8. Signaltime-scaling arrangement as claimed in any one of the preceding claims,wherein the signal-dispersion means comprises an optical fibre havingintrinsic dispersion properties.
 9. Signal time-scaling arrangement asclaimed in any one of the preceding claims, wherein thesignal-dispersion means comprises a fibre Bragg-grating device. 10.Signal time-scaling arrangement as claimed in any one of the precedingclaims, wherein the optical source is a laser.
 11. Analogue-to-digitalconversion arrangement comprising a signal time-scaling arrangement asclaimed in any one of th4 preceding claims, wherein the output of thesignal-dispersion means is coupled to an opto-electrical converter forconverting the optical signal at the output of the signal-dispersionmeans into an electrical signal, and an output of the opto-electricalconverter is coupled to an electrical analogue-to-digital conversionmeans.
 12. Analogue-to-digital conversion arrangement as claimed inclaim 11, wherein the opto-electrical convector is a photodiode. 13.Frequency-shifter arrangement comprising a signal time-scalingarrangement as claimed in any one of the preceding claims, whereinspectral components of the modulating signal appear shifted in frequencyat the output of the dispersion means.
 14. Frequency-shifter arrangementas claimed in claim 13, wherein the frequency-shifter arrangement is aDoppler generator arrangement in which the modulating source is a sourceof continuous sinewaves.
 15. Frequency-shifter arrangement as claimed inclaim 14, wherein the sinewaves are of a predetermined frequency or arewithin a predetermined range of frequencies.
 16. Method for time-scalinga signal, comprising: providing a source of optical radiation of awavelength which is varied with time over a given wavelength range;intensity-modulating said radiation with an analogue signal which is tobe time-scaled, the intensity-radiation Wing place in a single-sidebandmodulator, and subjecting the modulated radiation to a dispersionprocess whereby different wavelength components of the radiation aredelayed by different amounts, the thereby dispersion-processed modulatedradiation constituting the time-scaled signal.
 17. Method fortime-scaling a signal as claimed in claim 16, wherein the radiation iscontinuous-wave radiation whose wavelength is varied in a continuousmanner.
 18. Method for time-scaling a signal as claimed in claim 17,wherein the wavelength is varied in a substantially linear manner. 19.Method for time-scaling a signal as claimed in claim 18, wherein thesignal is time-stretched by arranging for the wavelength to be varied ina direction of increasing frequency and for the dispersion means tosubject the higher-frequency components of the radiation to a longerdelay than the lower-frequency components thereof.
 20. Method fortime-scaling a signal as claimed in claim 18, wherein the signal istime-stretched by arranging for the wavelength to be varied in adirection of decreasing frequency and for the dispersion means tosubject the lower-frequency components of the radiation to a longerdelay than the higher-frequency components thereof.
 21. Method fortime-scaling a signal as claimed in any one of claims 18 to 20, whereinthe signal is time-compressed by arranging for the wavelength to bevaried in a direction of decreasing frequency and for the dispersionmeans to subject the higher-frequency components of the radiation to alonger delay than the lower-frequency components thereof.
 22. Method fortime-scaling a signal as claimed in any of claims 18 to 20, wherein thesignal is time-compressed by arranging for the wavelength to be variedin a direction of increasing frequency and for the dispersion means tosubject the lower-frequency components of the radiation to a longerdelay than the higher-frequency components thereof.
 23. Signaltime-scaling arrangement substantially as shown in, or as hereinbeforedescribed with reference to, FIG. 4 or FIG. 5 of the drawings. 24.Analogue-to-digital converter arrangement substantially as shown in, oras hereinbefore described with reference to, FIG. 4 or FIG. 5 of thedrawings.
 25. Frequency-shifter arrangement substantially as shown in,or as hereinbefore described with reference to, FIG. 4 or FIG. 5 of thedrawings.